We use AWS Lambda heavily, and even when we don't it was difficult to find a full featured library to validate incoming requests to our service.
It's always critical to validate user inputs, but as an API first product it's also important for this to be done right. It has to match our api docs exactly or else users are going to complain.
That's why we are happy to release the open source request validator which utilizes any OpenAPI spec for validation: OpenApi Validator
Hope that helps the community, make great apis!